As the American population ages, bleeding on the surface of the brain (subdural hematoma) has become increasingly common. By 2030 subdural hematoma will be a more frequent surgical problem than both primary and metastatic brain tumors. In the 1960's only 1or 2 of 100,000 people was affected by subdural hematoma each year, but by 2005, that number grew to 20 of 100,000 people per year. In our patient population of adult veterans from 2005-2010, the incidence was 31.5 per 100,000 per year. People requiring surgery for subdural hematomas are often in the hospital for longer time than patients with brain tumors. Subdural hematoma patients may be disabled by their disease, and have a shortened life expectancy. It is possible that the increase in incidence of subdural hematoma is due to increased consumption of aspirin and other blood thinning medications. Older people and other populations at risk for brain atrophy (shrinkage) may be most at risk for subdural hematoma because veins on the surface of the brain are stretched and torn more easily in these patients. In this grant, we propose to demonstrate that the amount and pattern of brain atrophy impacts the risk of chronic subdural hematoma, and that consuming aspirin or blood thinners further compounds that risk. The purpose of this work is to establish that a screening tool that evaluates brain images and quantitates atrophy is useful for assessing risk prior to starting aspirin or blood thinners in people who are most at risk for such bleeding. We will study a cohort of 10,351 veterans who all had imaging studies of their brains performed during the years 2005-10 with a novel automated method for quantitating atrophy using standard CT scans of the head. We will also record whether or not these individuals consumed aspirin or blood thinners. We will follow the patients for four years and track whether they develop subdural hematomas, or other disorders related to brain bleeding or blood clotting. We will then perform statistical analyses to determine how much the risks of atrophy and consumption of aspirin and blood thinners separately and together impact the risk for development of subdural hematoma. The success of this project would enable prevention of an increasingly common, disabling and potentially fatal brain disorder that already impacts the lives of 60,000 Americans per year.